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Formation of frost on outdoor evaporators poses a significant challenge for air source heat pumps operating in cold climates. It is widely recognized that uneven air-flow across an evaporator can have a negative impact on both the evaporator's performance and the overall efficiency of the heat pump. This study investigates the influence of non-uniform air-flow distribution on a V-shaped outdoor evaporator as a component of an air-source transcritical CO 2 heat pump when subjected to frosting conditions. To this end, we developed and validated against independent experiments a numerical frost-formation model, and used it to study a multi-circuit evaporator of interest under different ambient conditions. It is shown that the uneven air-flow distribution results in varying frost formation patterns across different circuits of the heat exchanger, including differences in the amount of frost accumulated and its distribution among the individual tubes; the circuit with the largest frost weight accumulates 18% more frost compared to the one with the smallest in one hour under the sever frosting condition. Non-uniform frost formation also leads to air-flow distribution changing over time, and the rate at which this change occurs depends on the ambient conditions. For the presently investigated case, frost formation suppresses the non-uniformity of air-flow distribution. Moreover, evaporators with non-uniform and ideal uniform air-flows are compared and it is revealed that the difference in their capacities is more noticeable at the beginning of the frosting period. • Effect of non-uniform airflow on frosting behavior of a V-shape evaporator studied. • Different amounts of frost among circuits due to non-uniform airflow quantified. • Effect of ambient humidity on frost non-uniformity investigated. • Response of air-flow to frost formation over time investigated. • Drop in the evaporator capacity compared to the case of an ideal uniform airflow. [ABSTRACT FROM AUTHOR]

• The structural model was constructed for a water evaporator operating in a low-pressure environment. • A multi-objective optimization method combining CFD simulations and the MO-SHERPA algorithm was proposed. • The offset strip fin structure and heat transfer performance of the water evaporator was optimized. The aim of this study was to investigate the optimal heat transfer performance and structure of airborne water evaporators in low-pressure environments. To achieve this, a multi-objective optimization design was performed on a plate-fin water evaporator with offset strip fins, operating in a low-pressure environment at an absolute pressure of 7 kPa. The multi-objective Simultaneous Hybrid Exploration that is Robust, Progressive and Adaptive (SHERPA) (MO-SHERPA) algorithm was used to optimize the structure of the plate–fin water evaporator under a low pressure of 7 kPa. There are three conflicting optimization objectives, including comprehensive performance factor (JF), entropy generation (S gen), and weight. The optimal Pareto frontier design points were compared and analyzed based on the three evaluation indicators. The optimization results showed that compared with the initial design point, the JF increased by 32.14% and the weight decreased by 48%, which indicated that the heat exchange performance has been significantly improved, and the weight of the heat exchanger was also reduced. The internal and external flow fields of the water evaporator, including temperature, pressure, and volume fraction of vapor, were qualitatively compared to prove the optimization effect. In addition, the influence of structural parameters on the heat transfer performance of the water evaporator were analyzed. The method proposed in this study can effectively optimize the fin structural design parameters, providing valuable guidance for the structural design of water evaporators operating in low-pressure environments. [ABSTRACT FROM AUTHOR]

• A quantification method is proposed to calculate the flow rate distribution of microchannel heat exchangers. • Liquid and vapor mass flow rates are calculated with the method, and it shows how refrigerant is distributed in the inlet header. • The potential of improving the heat capacity of the evaporator with uniform distribution is calculated and discussed. Maldistribution of fluid among the ports reduces the heat capacity of a microchannel heat exchanger. It is essential to understand the maldistribution mechanism to optimize the design of the heat exchanger. Quantification of the distribution helps analyze the maldistribution issue. This study proposes a method to quantify the flow distribution and capacity potential in the microchannel heat exchangers by integrating a microchannel heat exchanger model and infrared thermography. The method calculates the liquid and vapor mass flow rate at the inlet of each microchannel tube by comparing the wall temperature of a heat exchanger from simulation and the infrared image. The method is validated with tests of a 49-tube horizontal microchannel evaporator with R134a. The exit region of the top tubes has an extremely high surface temperature, which forms a superheat region. More liquid flows to the tubes located at the center of the heat exchanger. The total mass flow rate is low when the tube has a long superheat region. The heat capacity of the tested evaporator has 9.6–21.3% improvement potential when uniformly distributed. When the refrigerant outlet superheats of the evaporator is lower, the improvement potential is higher. In the superheat region, the heat flux is much lower than the average value. This quantification method provides a tool for a more profound investigation of the maldistribution issue of a microchannel evaporator, and it can be expanded to heat exchangers with different structures. [ABSTRACT FROM AUTHOR]

• Systematic investigation of liquid flooding upon compressor start-up. • Characteristic flooding curve shown for 12 combinations of heat exchangers and refrigerants. • Correlation proposed capturing influence of heat exchanger size and refrigerant properties. Refrigeration cycles with fixed speed compressors cycle on and off to modulate their time averaged cooling capacity. While the cycles are turned off, refrigerant can migrate from the condenser into the colder evaporator. The next start-up process can then pull liquid refrigerant out of the evaporator and into the compressor causing liquid flooding. This phenomenon results in a loss of latent heat for cooling, can undercool the lubricant, and may cause damage to the compression mechanism. The geometric and thermophysical conditions that lead to liquid flooding are poorly understood. Experimental data collected for four refrigerants and three heat exchangers was analyzed for flooding conditions. A correlation was fitted to the experimental data, predicting the experimentally determined charge threshold for flooding with an average deviation of 6.6% and a maximum deviation of 16.7%. Repeating the tests with other heat exchanger types and smaller or larger compressors is suggested as important future work. [ABSTRACT FROM AUTHOR]

Due to a high solar energy utilizing efficiency, Direct-expansion Solar-Assisted Heat Pump (DX SAHP) system has been a research hotspot for many years, especially for the system using PV/T module as the evaporator. The heat transfer throughout the PVT evaporator is an important factor for either electrical or thermal performance. We have developed and analyzed numerically the temperature distribution of a multi-source PVT heat exchanger of serpentine shape to fit a function as evaporator/condenser for Direct-expansion Photovoltaic/Thermal-Heat Pump (DX PVTHP) system. The suitability of the developed model was based on the calculated results of the efficiency factor and simulated results of temperatures of heat exchanger and working fluid as well the working fluid pressure drop. During the numerical simulations, we have considered different temperature influencing factors such as the refrigerant and refrigerant flow rate, spacing between the adjacent tubes of the heat exchanger, backside convection and radiation effect and finned heat exchanger. Simulated results showed that the serpentine PVT heat exchanger with fins and small space between tubes (62.5 mm) at 0.02 kg/s flow rate could achieve better results in terms of uniform temperature distribution, smaller fluid pressure drop (0.078 bar) and the highest efficiency factor (0.9873) which make it recommended as the best option for suitable evaporator/condenser. The average PV temperature reached by cooled and uncooled PVT evaporator at 0.02 kg/s flow rate was 47.81 °C and 71.11 °C, respectively. It was 40.99 °C and 37.07 °C at the flow rate of 0.035 kg/s and 0.05 kg/s, respectively, and 44.44 °C for PVT evaporator with smaller space between tubes at 0.02 kg/s. The average PV temperature reached by the PVT evaporator with fins was 40.734 °C at the flow rate of 0.02 kg/s while it was 53.956 °C when the unit is insulated. [ABSTRACT FROM AUTHOR]

The heat exchangers are essential components in the CO 2 heat pump-driven liquid desiccant dehumidification system since the heat transfer performance of heat exchangers directly affects the energy efficiency of the system. However, the heat transfer characteristics of CO 2 and liquid desiccant in the heat exchanger have yet to be fully revealed, resulting in a lack of appropriate strategies to improve the heat transfer performance. To address this issue, numerical gas cooler and evaporator models are developed and verified. The effects of parameters such as the weight concentration of the CaCl 2 aqueous solution, the CO 2 inlet pressure, mass flow rates, and the length of the heat exchanger on the heat transfer performance are investigated respectively under various operating conditions. Results show that the heat exchange load of the gas cooler can be significantly impacted by various inlet parameters of both fluids. In particular, the increase in the weight concentration from 36 to 54 wt.% deteriorates the heat exchange load by over 15 %. Nevertheless, the heat exchange load of the evaporator is hardly influenced by the weight concentration. This is attributed to a great gap in the two fluids' heat capacity rates, causing the evaporator to be in the maximum heat transfer state. The sign of the maximum heat transfer state of the heat exchanger is whether the thermal effectiveness is close to 1, which is related to the heat transfer area. Therefore, modifying the heat exchanger length could avoid the thermal effectiveness approaching 1. Results indicate that the appropriate length of the evaporator is 3–4 m under the designed operating conditions. • Heat transfer process for CO 2 and CaCl 2 aqueous solution is simulated accurately. • The heat transfer performance in the gas cooler and evaporator is investigated. • The evaporator is in the maximum heat transfer state in the designed condition. • The appropriate lengths of the gas cooler and evaporator are 14 m and 3–4 m. [ABSTRACT FROM AUTHOR]

Fin-and-tube heat exchangers can have active circuits directly adjacent to inactive circuits at part load conditions, leading to cross-fin conduction. Cross-fin conduction needs to be accounted for in heat exchanger modelling to accurately predict heat exchanger performance. This study explores how refrigerant circuitry can influence cross-fin conduction in multi-circuit evaporator coils. Our advanced segment-by-segment heat exchanger model is validated against two-phase refrigerant data obtained from our experimental facility. Three, custom designed, multi-circuit fin-and-tube heat exchanger coils with distinct circuities were used for validation purposes, including an interleaved, vertical and block circuited coils, respectively. A total of 162 experiments with several refrigerant and air inlet conditions, at full load (all circuits active) and part load (some circuits active) were carried out. At full load, for all coils, differences in coil capacity between simulated and experimental capacities were no greater than 10%, for part load the differences are larger. For the test coil with interleaved circuitry, operating with 3 of 8 circuits active, the mean average percentage error (MAPE) between experimental and simulated capacity was 2.7% when cross-fin conduction was considered, increasing to 7.7% when it was ignored. Similarly, for test coil with block circuitry, operating with 2 out of 4 circuits active, the MAPE between experimental and simulated capacity was 1.7% when cross-fin conduction was considered, and 3.4% when ignored. Results suggest that the effect of cross-fin conduction on coil capacity is directly proportional to number of inactive circuits, and it is more pronounced on interleaved circuitry compared to block circuitry. [ABSTRACT FROM AUTHOR]

Part I showed that Carbon Dioxide (CO 2) is a promising candidate for use in vertical direct expansion ground evaporators, as it requires smaller pipe diameters within the geothermal heat exchanger and lower power for refrigerant circulation compared to conventional refrigerants. Furthermore, pressure change along the geothermal evaporator influences marginally the evaporation temperature leading to quasi-isothermal evaporation of CO 2. However, in single U-pipe configurations, small pipe diameter means lower pipe surface area and therefore higher thermal resistance. In Part II, all these advantages are taken into consideration toward proposing new geothermal evaporator configurations with multiple U-pipes (2 to 4 U-pipes in one borehole). R410A is also compared against CO 2 for the new configurations. Operating parameters of all configurations were chosen so that CO 2 and R410A are both subjected to a complete evaporation for equal total pipe surface area and equal heat extraction rate. Although all configurations have same total pipe surface area, it is shown that adding U-pipes and reducing pipe diameters diminishes borehole thermal resistance significantly. A novel configuration is also proposed to lower pressure drop of CO 2 in the 4-U borehole configuration by insulating a portion of the downward U-pipe legs. Decreasing pipe dimensions by adding U-pipes while keeping total pipe surface area constant offers an opportunity to bundle pipes closer to each other and make the evaporator more compact as seen in the proposed Polymer Block configuration. [ABSTRACT FROM AUTHOR]

CO 2 is a promising candidate for replacing synthetic refrigerants used in direct-expansion ground-source heat pumps (DX-GSHPs). The present paper, as the first part of two, presents a numerical performance comparison of a vertical single U-pipe direct expansion geothermal evaporator using different refrigerants; namely R744, R410A, R22, R407C, R12345yf, and R134a. Each refrigerant was tested through a borehole heat exchanger (BHE) performance simulation at a constant rate of heat extraction. Operating parameters were chosen so that each refrigerant is subjected to a complete evaporation cycle, and a similar pressure drop from the inlet to outlet of the borehole. Several BHE parameters such as fluid temperature, pressure, mass flow rate, required pipe dimensions, and required power for fluid circulation were evaluated for different refrigerants. Finally, the overall performance of each refrigerant was characterized by the heat exchange capacity for a unit of mass flow rate and pipe surface area. Results showed that CO 2 offers unique characteristics favoring the design of compact BHEs, as it required the smallest mass flow and pipe surface area. It is also concluded that the performance of a BHE with CO 2 can be significantly improved through using unconventional configurations. This will be discussed further in the second part (PART II). [ABSTRACT FROM AUTHOR]

Desiccant coated heat exchangers (DCHEs), utilizing an inner cooling source to remove sorption heat, are promising alternatives for evaporators and condensers (DCEs/DCCs) in vapor compression (VC) heat pumps. A mathematical model is necessary to facilitate the design, analysis and performance prediction of the component and the relevant systems. In this study, a three-dimensional model of DCEs/DCCs is proposed, accounting for the two-phase phenomena, the periodical switchover, the solid-side resistance, the fluid transport in multiple directions and the coupled heat and mass transfer. Study reveals that, high latent load (inlet humidity) reduces the sensible heat handling capacity of the DCEs, while the dehumidification capacity is almost independent of its sensible heat load. DCEs possess a satisfying effect of dehumidification above the dew point, thus it is unnecessary to employ low evaporation temperature. Meanwhile, the adsorption-desorption temperature difference of 30°C seems to be the optimal value for the commonly adopted evaporation temperatures (10°C-20°C). For a specific coating thickness, there is a critical switchover period where the best performance of dehumidification is obtained. Switchover period shorter than the critical time should be avoided, and elongating the switchover cycle helps balance the ability of the system to handle the sensible and latent heat loads at the same time. [ABSTRACT FROM AUTHOR]

Highlights • A new Constructal law-based design is presented for cross-flow heat exchangers. • Compared to cross-flow design, the thermal performance is increased as much as 17%. • Compared to cross-flow design, the cost is decreased as much as 14%. • The method is computationally very efficient and can be used in preliminary design. Abstract With recent advancements in advanced manufacturing techniques, it is now possible to fabricate complex geometries that take advantage of well known principles of heat transfer. Therefore, unconventional configurations to enhance effectiveness beyond conventional designs can now be considered for practical application. Thermal performance and overall cost of a new design of heat exchangers in counter cross-flow configurations are studied using a simplified but accurate computational method. The new heat exchanger design was introduced and studied previously for a cross-flow configuration by Sabau et al. (2016, 2018). This new design concept uses multi-scale configurations with successive plenums for the working fluid. At the smallest scale the tubes are sized to be equal to the hydraulic entrance length of the inside fluid, in accord with constructal design. Results indicate that compared to the earlier cross-flow configuration, the counter cross-flow arrangement improves the thermal performance of the heat exchanger by as much as 17% and lowers the total cost by as much as 14%. [ABSTRACT FROM AUTHOR]

Graphical abstract This paper presents an experimental investigation of heat transfer performances including heat transfer coefficient along flowing direction, log-mean temperature difference and differential pressure of pure R123 and four volume concentrations 20 nm Al 2 O 3 -R123 nanorefrigerants with various volume concentrations 0.03%, 0.13%, 0.18%, 0.23% flowing inside a double-tube counter flow heat exchanger of an organic Rankine cycle system under the conditions of various heat source temperatures and flow rates. A comparison has been made between pure R123 and four kinds of nanorefrigerants to find out the influences of different nanoparticles volume concentrations on local heat transfer coefficient and variation tendency (straight lines) along flowing direction in evaporator. By using polynomial fitting where polynomial order is 3, the polynomial fitting curves (dash lines) and the optimum of volume concentration has been acquired, which indicates the loading carrying capacity in each operation condition. Highlights • Effect of Al 2 O 3 nanoparticles on R123 heat transfer performances. • Variation tendency along flowing direction in evaporator of ORC. • Polynomial fitting curve. • Comparison of differential pressure and log-mean temperature difference of R123 and nanorefrigerants. Abstract This paper presents an experimental investigation of variation tendency of heat transfer coefficient, log-mean temperature difference and differential pressure of pure R123 and 20 nm Al 2 O 3 -R123 nanorefrigerants with four various volume concentrations, 0.03%, 0.13%, 0.18%, 0.23% flowing inside the evaporator of organic Rankine cycle system under the conditions of various heat source temperatures and flow rates. Heat source temperatures are in the range of 50–90 °C at an interval of 10 °C, and heat source flow rates are 0.7 m3/h, 1.3 m3/h and 1.8 m3/h. Results show an increment of heat transfer coefficient along flowing direction for both pure R123 and four Al 2 O 3 -R123 nanorefrigerants with the increment of heat source temperature and flow rate, and that of four nanorefrigerants are higher than that of pure R123. There is no optimum value of heat transfer coefficient when operation condition is changed for the loading carrying capacity increasing with the increasing intensity of operation condition. Meanwhile, suspending nanoparticles and increasing heat source temperature can change the variation tendency of heat transfer coefficient along flowing direction except pure R123 and 0.18 vol% nanorefrigerant. In addition, 0.13 vol% as a whole is the optimum volume concentration for both log-mean temperature difference and differential pressure. [ABSTRACT FROM AUTHOR]

Highlights • A numerically efficient but accurate model for frosting evaporators is developed. • The enthalpy-based reformulation and linearization method are employed • Simulation results on flat plate and finned-tube heat exchangers are provided. • It is 8 to 20 times faster than reference models but withcomparable accuracy. Abstract Growth of a frost layer on an evaporator surface due to low evaporator temperature as well as moisture contained in surrounding air deteriorates performance of a refrigeration system significantly and requires significant energy for defrost. Many studies have been performed to model the heat and mass transfer phenomena in an attempt to have insight and accurate prediction. However, many models form nonlinear algebraic differential equations which require iterative numerical solvers. Computationally efficient but accurate models are needed in order to evaluate overall system performance. The objective of this paper is to introduce a modeling approach to overcome the problem. A solution strategy based on an enthalpy-based reformulation and linearization method will be presented. Comparisons of the proposed and detailed model results for both flat plate and finned tube heat exchangers are provided. The proposed modeling approach is around 10 times faster than reference models while maintaining comparable accuracy. [ABSTRACT FROM AUTHOR]

Highlights • A method for the determination of defrosting start time is proposed. • Frost volume and blocking ratio are calculated from experimental data. • Frost volume calculation method is verified by image processing of the evaporator. • This method accurately determines the timing of deterioration of the system. Abstract Naturally, frost forms at fin-tube evaporator surface when refrigerant below 0°C flows inside the heat exchanger. Frost tends to block the evaporator, hinders efficient heat exchange and degrades the performance of the system. The determination of the start time for defrosting is one of the essential issues that most of air heat pumps are confronted with because it is directly connected to energy consumption. Many types of sensors can be installed for frost detection but they are usually not cost-effective or related logic is not so physically presented. In this study, a novel frost volume calculation method is suggested in order to determine the start time for defrosting in an air-to-air heat pump system. The detailed explanation of the method is presented and verified with experimental data. With this method, the start timing of defrosting can be appropriately judged. At the time of defrosting, the heating capacity decreased from 16.2 to 39.3% and the COP reduced from 12.3 to 26.4% compared with maximum values. The blocking ratio from image processing at the time was between 0.83 and 0.97. [ABSTRACT FROM AUTHOR]

Understanding the bubble regimes is a fundamental step toward conducting heat transfer enhancement. The non-invasive measurement of mixing inside a direct-contact heat transfer process, using a direct video imaging technology, provides powerful opportunities for characterising the visual observations of the phenomena and quantifying the process complexities previously. Experimental bubble shape feature parameters were obtained by means of the photographic recording technique for a direct-contact evaporator. Four design factors with three levels respectively were analysed for the mixing system that involves the exchange of heat between two immiscible fluids (continuous and dispersed phases). Using the Ripley’s K function, new results are presented for two-phase flow mixing which can distinguish differences in the mixing behavior of dispersed phase. In all cases considered, quantitative comparisons of the evolution curves representing different experimental conditions were conducted with reported experimental data. Following the local mixing curve, the current results can also be processed to provide the mixing time found to be in good agreement with available data. The relationship between shape feature parameters of bubbles and volumetric heat transfer coefficient was found to be highly independent on experimental design parameters. [ABSTRACT FROM AUTHOR]

• Heat exchangers in which phase changes occur require simulation models that realistically capture the resulting temperature profiles and heat-transfer rates. • Two industrially important process examples are studied: a steam generator heat exchanger and the two heat exchangers in a compression refrigeration system. • The reality of the process is compared to the approximate simulation implementation. • Effective control structures are developed and tested. The heat exchanger is probably second only to the pipe as the most widely used piece of process equipment. A typical chemical plant has hundreds of heat exchangers that transfer heat from a hot source into a cold sink. The analysis of heat transfer is straight-forward when both the hot and the cold streams do not undergo any change in phase during the heating and cooling. However, the analysis gets more complex when there is vaporization or condensation of either of the streams. Now latent heats are involved and temperatures do not change continuously with axial location. Vaporizers, reboilers, evaporators and condensers are common examples of heat transfer with phase changes. The purpose of this paper is to demonstrate how realistic simulation models can be developed that capture the steadystate and dynamic heat-transfer differential temperature driving forces, vapor-liquid fractions and heat-transfer rates for these important and widely applied exchangers. [ABSTRACT FROM AUTHOR]

• A novel evaporator for R744 heat pumps with wastewater as heat source was presented. • This evaporator splits the evaporation in two levels, connected by an ejector. • This evaporator was tested and compared to one-stage heat exchangers. • Heat pump COP can improve more than 10 % compared to the benchmark evaporators. Heat pumps are pinpointed as a key technology in decarbonizing buildings' heat supply, which is currently dominated by fossil-fuel based systems. To minimize the environmental impact of heat pumps over their lifetime, selecting natural working fluids (refrigerants) is the only viable long-term choice. Independent of the refrigerant, the efficiency of a heat pump is closely linked to temperature of the applied heat source, and to this end, wastewater is a promising candidate that is abundant in all urban environments. This experimental study investigates a novel two-stage, ejector-supported, brazed plate evaporator for its use in R744 (CO 2) heat pumps with greywater as a heat source and compares it to more conventional evaporators. The work shows that splitting the evaporation process in two stages with an ejector in between allows reducing the evaporation pressure while the compressor suction pressure can be increased, in some cases significantly. Even if several possibilities for further optimization of the novel two-stage evaporator were identified, the results show heat pump efficiency improvements above 10 % compared to an equivalent heat pump with a well-performing one-stage evaporator. Heat pump systems aiming at utilizing maximum temperature difference for the greywater should use a high pressure lift ejector. [ABSTRACT FROM AUTHOR]

This paper presents a derivation of design guidelines for plate heat exchangers used for evaporation of zeotropic mixtures in heat pumps. A mapping of combined heat exchanger and cycle calculations for different combinations of geometrical parameters and working fluids allowed estimating the trade-off between heat transfer area and pressure drops on the thermodynamic and economic performance indicators of the cycle. Compressor running costs constituted the largest cost share, and increased due to a steep decrease of the heat pump coefficient of performance at high refrigerant pressure drops. It was found that the pressure drop limit leading to infeasible designs was dependent on the working fluid, thereby making it impossible to define a guideline based on maximum allowable pressure drops. It was found that economically feasible designs could be obtained by correlating the vapour Reynolds number and the Bond number at the evaporator inlet as Re V − 0.42 Bd 0.26 ≈ 0.040 . The use of the proposed guideline was illustrated for the mixture Propane/Iso-Pentane (0.5/0.5), leading to evaporator designs with net present values deviating maximum −4.4% from the best value found in the mapping. The presented methodology can be applied in different scenarios to develop similar guidelines, thereby decreasing the cost of combined cycle and component optimizations. [ABSTRACT FROM AUTHOR]

Heat exchangers are common components of many industrial, residential, and automotive systems. In automotive, heat exchangers are employed in engine cooling and air conditioning to transfer the undesired heat from the engine or the passenger compartment as a condenser, evaporator, inter-cooler, heater core, radiator, and oil cooler. Since, the thermal performance of heat exchangers influences the underhood automotive thermal management system, it is essential to characterize their dynamic response in general and specifically, when a sudden change to their operating parameters occur. Experimental measurements provide the scope for in depth understanding of the dynamic behavior of heat exchangers with respect to various parameters. This study investigates a liquid to gas cross flow meso heat exchanger subjected to transient conditions. Step variations in the hot fluid inlet temperature in the absence of mass flow rate disturbances was considered for 5 different levels ranging from 1.5 to 3.0, while other operating parameters are kept constant. The effect of step change on the thermal performance of a heat exchanger is demonstrated by both fluids outlet temperatures, heat transfer rate, effectiveness, and heat balance error. Results obtained show a faster response of the cold fluid outlet temperature compared to the hot fluid, however, an adverse effect is noticed for the heat transfer rate. Higher step changes in temperature lead to higher heat transfer rate, as expected; though, the hot side exhibited more heat transfer and higher effectiveness than the cold side. The hot fluid heat transfer rate and effectiveness display a non-linear increase reaching a peak, after which, it drops down at a slower rate compared to the cold fluid steady increase. A significant effect on normalized exit temperatures, heat transfer, and effectiveness at low step changes while it diminishes at higher step changes. The key element of this work is the use of meso heat exchanger to examine the transient conditions effect on its performance. This work covers a wide range of temperature step changes and aims to enrich the limited experimental database of dynamic response of heat exchangers. The outcome of this work is of interest to designers for thermal testing, characterization, and performance enhancement of compact heat exchangers. [ABSTRACT FROM AUTHOR]

This study is developed to evaluate the possibility of R1234yf to be a drop-in replacement for a pre-designed system with R134a, including the characterization of control system in a vapor compression system. The control algorithms are required for regulating the refrigerant flow rate into the evaporator and their control parameters depend intrinsically on refrigerant fluid. This paper presents a comparison of the effects of R134a and R1234yf refrigerants on the transient response of an evaporator operating with water as secondary fluid (counter-flow). The dynamic heat exchanger distributed model combines principles of thermodynamics, heat and mass transfer with empirical correlations, which are proposed in the literature, in order to compute the void fraction, pressure drop and heat transfer in two-phase flow. Experimental tests are carried out on a fixed refrigeration system for cooling capacity less than 3 kW, with constant condensation conditions, variable evaporating temperature and compressor speed to validate the model. The results demonstrated that a good agreement between simulation results and experimental data was achieved. The evaporation temperature data were within ± 1   °C error band. Furthermore, comparative simulations between the refrigerants R-134a and R-1234yf exhibited a similar dynamic behavior during a step at the inlet mass flow rate, and the gain and time constant for superheat control were not considerably modified. Therefore, the refrigerant changing does not demand many changes in the expansion device control strategy. [ABSTRACT FROM AUTHOR]

Performance of a frost-free refrigerator may be deteriorated by frost accumulation on its evaporator surface. The objective of this study is to investigate the feasibility of using an enthalpy exchanger (EE) to reduce the frost accumulation on evaporator surface in frost-free refrigerator. The influence of EE on air circulation and the change process of air humidity is analyzed, and it is indicated that the application of EE is capable to decrease the amount of water vapor entering the evaporator and consequently reduce the frost accumulation on evaporator surface. The design process of EE in frost-free refrigerator is illustrated, and an optimal type of EE is presented. An experiment on a frost-free refrigerator shows that the application of EE decreases the frost accumulation by 18.8%. In addition, effects of EE on other main performances of frost-free refrigerator besides the frost accumulation are evaluated. [ABSTRACT FROM AUTHOR]

In this paper, a novel geometry is proposed for evaporators that are used in Supercritical Organic Rankine Cycles. The proposed geometry consists of successive plenums at several length-scale levels, creating a multi-scale heat exchanger (HX). The channels at the lowest length-scale levels were considered to have their length determined by the thermal entrance-length. Numerical simulations based on turbulent flow correlations for supercritical R134a and water were used to evaluate the performance of heat exchangers. Using the data on pumping power and area of heat exchange, the total present cost was evaluated using a cost model for shell-and-tube heat exchangers. With respect to the shell-and-tube baseline case, the cost per heat load and total costs of new HXs is lowered by approximately 20–26% and 15–30%, respectively. This reduction in present costs of the new HXs were found to be attributed to higher operational costs for the shell-and-tube HXs, as evidenced by the higher pumping power, as well their capital investment costs. The cost savings in the new HX designs compared to those of the shell-and-tube HXs, at similar heat load performance, indicate that the new HX architectures proposed in this paper are valid alternatives to traditional HX designs. [ABSTRACT FROM AUTHOR]

The present study focuses on the energetic optimization of regenerative Organic Rankine Cycle (ORC) configurations. More specifically, three regenerative ORCs are examined. The first includes an open preheater, in which the bleed stream is mixed with the working fluid exiting the pump of the cycle (O-ORC). The other two configurations include a closed preheater. In the second configuration, the bleed stream is throttled and conveyed to the condenser (CB-ORC), while in the third one, it is repressurized via a secondary pump and recirculates into the evaporator of the cycle (CF-ORC). The systems are optimized for different working fluids, and their energetic efficiencies are estimated and compared to that of a standard ORC (S-ORC). In all cases, the inclusion of a recuperator has also been investigated. In principle, recuperative and regenerative ORCs are mostly suitable for dry fluids, while the critical temperature can also have a positive influence on the performance improvement. Furthermore, it is estimated that while the recuperative S-ORC has a higher efficiency than the non-recuperative regenerative cycles, recuperative O-ORC and CF-ORC exhibit a relative efficiency gain ranging from 4.98% to 8.05% and 6.22% to 9.29%, respectively. The highest efficiency improvement achieved by the CB-ORC, however, is minimal. [ABSTRACT FROM AUTHOR]

This study presents an experimental and theoretical evaluation of frost formation on the moist air side of a peripheral finned-tube (PFT) heat exchanger. Previous studies of this compact geometry correlated the friction and heat transfer parameters above the dew-point temperature of the air (no frost or condensate formation). Here, for the first time, the thermal-hydraulic performance of a PFT heat exchanger is analyzed under frosting. A PFT heat exchanger prototype was evaluated experimentally in a closed-loop wind tunnel calorimeter to determine the influence of the tube wall temperature, air velocity and psychrometric properties (temperature and relative humidity) on the heat transfer rate, air-side pressure drop and frost buildup on the surface. The thermal and hydrodynamic behavior of the enhanced air-side surface was analyzed using a distributed heat exchanger model in which mass, momentum and energy balances are applied to one-dimensional control volumes in the air flow direction. The model treats the air flow path as a porous medium in which the porosity, equivalent particle diameter and thermal properties vary as a function of time due to the frost accumulation. Good agreements (within 20% average error) between the model predictions and the experiments for the air-side pressure drop and heat transfer rate have been found. The enthalpy effectiveness was found to drop from 0.68 for dry to 0.50 under severe frosting, which suggests that the PFT heat exchanger continues to be effective under frost. [ABSTRACT FROM AUTHOR]

Boiling instability and periodic flow reversal are commonly seen in microchannel heat exchangers, but have been studied in air conditioning systems only recently. The amount and frequency of reverse flow largely depends on the geometry of the channels. This paper presents results obtained in a R134a automotive air conditioning system operated under flash gas bypass mode with venting of reversed vapor (to provide better control of the flow). Results for multiple microchannel evaporators with different channel diameters and lengths are presented. The mass flow rate and frequency of reversed vapor are measured for each geometry. Meanwhile, a mechanistic model is developed to simulate the bubble dynamics inside of a single microchannel tube. This model is validated against experimental data and employed to explain the effect of channel length and diameter on flow reversal. Both experimental and simulation results show that larger channel diameter and longer channel length result in less flow reversal with a lower frequency. [ABSTRACT FROM AUTHOR]

Periodic flow reversal are commonly present in microchannel heat exchangers, but have been studied in air conditioning systems only recently. This paper presents the effect of periodic reverse flow on the heat transfer performance of a heat exchanger. Two heat exchangers with identical geometries in the heat transfer areas are employed and an artificial upstream flow resistance is added for one of them. The heat exchanger without artificial flow resistance is subject to more severe boiling instabilities and consequently generates four times more reverse vapor flow than the other one. The comparison of capacities under identical operating conditions reveals that higher intensity of reverse flow helps to improve cooling capacity by up to 13.3%. Meanwhile, numerical simulations of bubble dynamics coupled with heat transfer are carried out for both heat exchangers. Results show that in the heat exchanger with more reverse flow, the refrigerant side heat transfer coefficients are enhanced, especially in the upstream part of a channel where the flow velocity is relatively low. [ABSTRACT FROM AUTHOR]

Reducing the diameter of copper tube is beneficial to reduce the amount of metal material used in heat exchanger. The change of the diameter of the copper tube can not only reduce the volume of the heat exchanger, but also the filling amount of the refrigerant can be reduced. In this paper, the R22 fin-tube evaporator with 7mm tube diameter is improved. The diameter of the tube is reduced to 5mm, and the environment protection refrigerant R290 is used. Diameter reduction may cause the pressure drop increases, and the new branch number is 5 by calculating. Using the simulation software EVAP-COND to simulate the heat transfer performance and compare the refrigerant flow, pressure drop, copper usage with the evaporator before improved. Result show that the heat transfer effect of R290 evaporator is better than that of R22 system, and the heat transfer effect of system is the best. Under the premise of no increasing pressure drop, the heat transfer of the R290 fin-tube evaporator with a small tube diameter increased by 34.62%, refrigerant flow rate, pressure drop, copper and aluminum consumption decreased by 28.7%, 43.1%, 28.6%, 30%. [ABSTRACT FROM AUTHOR]

Evaporators for adsorption heat pumps, chillers and storage devices mostly use water as refrigerant and work at sub-atmospheric pressures. However, there are hardly any applicable performance correlations or sizing guidelines for these rather unusual operating conditions. Within this study geometric and process-related impacts on non-stationary evaporation performance of copper tube-fin heat exchangers are investigated to start filling that gap. Cyclic condensation/evaporation measurements were performed in a thin film evaporation mode and in partially flooded operation with changing refrigerant filling level to cover different applications. For thin film evaporation a thermal resistance model was developed to quantify resistance contributions and identify performance-limiting factors. The presented measurement results reveal that evaporation performance in thin film operation is crucially governed by fluid side heat transfer (UA raise by 146% within tested Reynolds numbers) and wetting conditions while fin sheet thickness plays a marginal role (8% increase of UA). In partially flooded operation performance strongly depends on filling level. Apart from some refinement potential of the thin film model, the simulations reflect evaporation dynamics and effects of influencing factors fairly well which indicates that the employed model approach could be a suitable tool for an effective optimization of evaporator geometry and process parameters. [ABSTRACT FROM AUTHOR]

Due to the lack of knowledge about water behavior in conditions that might occur in sorption chillers using water as refrigerant, designing compact heat exchangers remains mainly empirical. The objective of this paper is to gain further understanding of phenomena occurring in compact plate-type evaporator in order to be able to design them properly. In that goal, experiments were conducted under various operating conditions during adsorption cycles. Although interactions take place between the adsorber and the evaporator due to coupling phenomena, the present study focuses only on the evaporator. The vaporization of water occurring inside the channel of a smooth plate-type evaporator during these cycles is observed. Two main distinct phenomena observed along the time of the adsorption cycle are identified, described and discussed. Time-averaged cooling capacity and instantaneous cooling capacity for given operating conditions are plotted. A parallel with result obtained with the same experimental apparatus but in absorption configuration (mimicked by a thermosiphon loop) is done. Model developed in absorption configuration is tested. It is shown that, except during the beginning of the cycle, for the major part of running conditions tested, this model is able to well predict the instantaneous cooling capacity obtained during the adsorption cycle as long as the variation of the height of liquid level and the equilibrium pressure is known. The evolution of the cooling capacity measured in adsorption configuration with dimensionless number developed in absorption configuration is plotted for comparison. These dimensionless numbers were developed in order to optimize the design of compact evaporator used in sorption chillers. It is shown that the same trend could be observed for the two configurations. [ABSTRACT FROM AUTHOR]

The cost of heat exchangers account for a large proportion of total investment in organic Rankine cycle (ORC). In this paper, plate heat exchanger (P), shell-and-tube heat exchanger (S) and finned-tube heat exchanger (F) are used as evaporator and condenser of four subcritical ORC configurations: ORC-PP, ORC-SS, ORC-FP and ORC-FS. The thermo-economic models are built and a thermo-economic evaluation and comparison of four ORC configurations is presented in order to recover the low-temperature waste heat. The optimal evaporating pressure, pinch point temperature difference, net power output and dynamic payback period corresponding to the minimum electricity production cost (EPC) are obtained for different ORC configurations under different heat source temperatures. Results show that the EPCs of ORC-PP and ORC-SS are apparently higher than that of ORC-FP and ORC-FS. Among them, ORC-FS is the most cost-effective configuration. The optimal pinch point temperature difference in evaporator has a decreasing trend with the increase of critical temperature of working fluid for ORC-FS and ORC-FP, while the optimal pinch point temperature difference in condenser keeps nearly constant. [ABSTRACT FROM AUTHOR]

This study explores the implementation of two large micro-channel heat exchangers as evaporators in a refrigeration cycle intended for thermal management onboard future space vehicles. Examined are thermal performances of the two heat exchangers, thermodynamic performance of the refrigeration cycle, and system instabilities. Because of quality differences, different two-phase flow patterns and dryout effects are encountered in the two heat exchangers. A trade-off exists between optimum performances of the evaporators and compressor, with former favoring outlet quality values below unity to guard against dryout, and the compressor favoring a quality equal to or exceeding unity to preclude internal damage to the compressor from entrained droplets. Thermodynamic analysis of the refrigeration cycle shows exergy destruction is minimum when the avionics HX’s outlet quality is close to unity. The system is shown to be susceptible to instability resulting from interactions between compressible volumes both upstream and downstream of the condenser, and inertia of liquid in the condenser. This instability is manifest by pressure oscillations that are sensitive to both flow rate and evaporator heat input. [ABSTRACT FROM AUTHOR]

R-410A distribution was experimentally studied for an evaporator having two row/four pass configuration. The evaporator has inlet, intermediate and top (or bottom) row-crossing headers. Tests were conducted for the mass flux from 70 to 130 kg m −2 s −1 with inlet quality 0.2 and exit superheat 5 °C. Flow distribution at the inlet header generally follows the well-known trend. For downward configuration, more liquid flows into upstream channels. For upward configuration, more liquid flows into downstream channels. At the intermediate header, flow distribution is dependent on the direction of the tubes. When tubes are heading upward, more liquid is supplied into upstream. When tubes are heading downward, on the contrary, more liquid is supplied into downstream channels. Flow distribution at the top row-crossing header is entirely different from that at the bottom row-crossing header. Thermal degradation by flow mal-distribution is much larger for top row-crossing header configuration than for bottom row-crossing header configuration. Correlations are developed to predict the fraction of liquid or gas taken off by downstream channel as a function of header gas Reynolds number. Header pressure drops are obtained by subtracting flat tube pressure drops and other minor pressure drops from measured pressure drops. [ABSTRACT FROM AUTHOR]

Now-a-days fin-and-tube heat exchangers are employed in a wide variety of engineering applications such as modern heat exchangers, automotive radiators, automotive air conditioning evaporators and condensers. In order to improve the performance, further innovative designs, additional applications, and advanced geometries are expected. Researchers and academicians had been working on this topic for last few decades. A number of experimental and numerical studies have been conducted and various correlations were developed. The optimum design parameters with the maximum overall heat conductance and minimum pressure drop needs more focus and research. In this paper, different experimental/numerical studies performed are reviewed, grouped and summarized based on the types of heat exchangers, heat transfer and pressure drop performance, effects of geometrical parameters under different flow conditions. Overall, this paper will highlight on the existing technologies and emerging trends in designing of finned-tube heat exchangers considering different arrangements and geometric parameters under variable flow conditions which will be helpful for selecting appropriate design depending on the requirement. [ABSTRACT FROM AUTHOR]

• ORC and compound ejector-multi evaporator vapour compression systems are combined. • Nine ultra-low global warming potential refrigerant pairs are proposed to replace R134a and R245fa. • Condenser waste heat recovery increases the overall coefficient of performance. • The pair R1234ze(Z)/R1234ze(E) results in the highest energy performance. This article evaluates the energy and exergy performance of a novel dual fluid combined organic Rankine-compound ejector multi evaporators vapour compression system (ORCEMES) for power, cooling and heating purposes. Six working fluids with ultra-low global warming potential: R1234ze(E), R1243zf, R1234yf for the CEMES and R1234ze(Z), R1336mzz(Z) and R1224yd(Z) for the ORC were selected, resulting in nine combinations. The system can work in two operating modes: power-cooling and power-heating modes. The combination of R1234ze(Z) and R1234ze(E) results in the highest overall system energy performance. The proposed system increases power generation from 21% to 75% at high geothermal and low geothermal temperatures, respectively, compared with separated basic ORC and multi-evaporator systems at the same operating conditions and cooling capacity. The proposed CEMES reduces compressor power consumption to 85% of the basic system, increasing COP remarkably. Concerning the exergy analysis, the low-temperature recapture heat exchanger shows the highest exergy destruction compared to the rest of the components, followed by the turbine. Besides, the second expansion valve presents the lowest exergy destruction percentage. [ABSTRACT FROM AUTHOR]

Boiling heat transfer of R134a flowing in the inner tube and hot water flowing in the annulus were examined inside smooth and corrugated tube-in-tube heat exchangers in authors' previous studies experimentally. The apparatus has 0.85 m long double tube for vertical orientation and 2.5 m long double tube for horizontal orientation as its test sections whose inner ones are smooth and corrugated copper tubing having inner diameters of 0.0087 m for vertical tubes and 0.0081 m for horizontal tubes. The range of mass fluxes are 20,300 and 400 kg m − 2 s − 1 for vertical tubes, and 300, 400 and 500 kg m − 2 s − 1 for horizontal tubes. The average vapor qualities vary from 0.14 to 0.86 for vertical tubes, 0.09–0.81 for horizontal tubes, and saturation pressure interval is between 4.5 and 5.7 for both tube orientations. The average boiling heat transfer coefficient of R134a is calculated by means of the energy balance in the test section. In this existing study, the predictability of almost whole well-known empirical correlations in literature suggested for convective boiling flows in smooth and corrugated tubes is experienced by using the authors'large number of experimental database (302 data points for vertical tubes and 227 data points for horizontal tubes). Characteristics of trend lines plotted for the change of vapor quality, saturation temperature, mass flux and heat flux with experimental boiling heat transfer coefficients are examined as well in relation to various saturation temperatures and mass fluxes. Furthermore, the most successful correlations validated their predictabilities for both vertical and horizontal oriented evaporators having smooth and corrugated tubes. [ABSTRACT FROM AUTHOR]

The paper presents a semi-empirical model to predict refrigerant and lubricant inventory in both microchannel condenser and plate-and-fin evaporator of an air conditioning system. Validated against experiments, where mass of refrigerant and lubricant are isolated under steady state and afterwards measured within ±1% and ±2.5% uncertainty, respectively, the model predicts refrigerant mass in both heat exchangers within 20%. As for lubricant, initial application of the model predicts mass well for evaporator but consistently under-predicts for condenser. Analysis shows that the lubricant might be separated in the condenser inlet header. Accumulated in the bottom, the lubricant-rich liquid may start to fill in the microchannel tubes. The temperature profile in the infrared image supports this hypothesis, as the temperature of the bottom channels is much lower. After modifying the model by counting in these liquid filled channels, both refrigerant and lubricant mass in the condenser can be modeled within 15% error. [ABSTRACT FROM AUTHOR]

Diesel evaporators are one of the key components of diesel fuel processing for fuel cells. Diesel evaporator is required to evaporate diesel fuel in diesel fueled fuel cell systems. In this study, diesel evaporator which is a spiral-tube heat exchanger via indirect heat supplied by hot nitrogen gas is modeled and the fractional distillation curve results of diesel fuel are compared to experimental data reported in literature. In this work, the ANSYS Fluent 14.0 Computational Fluid Dynamics (CFD) code is used to simulate the 3-dimensional, turbulent, two-phase, multi-component and reacting flow-field, developed in a diesel evaporator. [ABSTRACT FROM AUTHOR]

Two models of a flat disk-shaped evaporator of a loop heat pipe have been developed for analyzing the effect of the convective component on heat transfer in a wick, and also on heat-exchange processes in an evaporation zone. Simulation data were obtained for two evaporators. The first evaporator was made entirely of copper, and the second had a body of stainless steel and a nickel wick. The geometrical dimensions of the evaporators were equal. The diameter of the heating zone was 30 mm. Calculations were made for heat fluxes from 2.8 ⋅ 10 4 to 4.2 ⋅ 10 5 W/m 2 . Water was used as a working fluid. An analysis of the results has shown that the contribution of the convective component to the overall heat transfer is small. The higher the thermal conductivity of the wick, the smaller the effect that the convection has on the temperature distribution in the evaporator. [ABSTRACT FROM AUTHOR]

Based on simulation model of the sewage source heat pump system (SSHPS), weigh the influence of coefficient of performance under different work conditions, the ACOP was defined and as the optimization function, with the heat exchange areas of the evaporator, condenser and the sewage-midwater heat exchanger as the optimization variables. The optimization mathematical models for the on-off control scheme were set up to calculate the ACOP to study the optimal design and areas matching between the evaporator, condenser and sewage-midwater heat exchanger. Comparison with the initial design parameters and different optimization designs with HSPF, SEER and EER, the ACOP optimization is nearer to the objective energy conservation effect. [ABSTRACT FROM AUTHOR]

Refrigerant R-410A distribution in a two pass evaporator with intermediate header was investigated. The number of tubes was 10 for the first pass and 12 for the second pass. Tests were conducted for the mass flux from 73 to 143 kg m −2  s −1 , quality from 0.4 to 0.6. In the intermediate header, two-phase mixture out of the first pass is merged and then re-distributed to the second pass. More liquid is forced downstream as mass flux or quality increases yielding better flow distribution. Effect of insertion device in the inlet header was also investigated. Efforts were made to develop correlations to predict the liquid or gas distribution in a header with limited success. Header pressure drop data are also provided. [ABSTRACT FROM AUTHOR]

A failure incident of heat-exchange tubes in ammonia evaporators, which suffered from unexpected wall thinning after only one-year service with respect to their original design lifetime of fifteen years, was reported and carefully analyzed. After overall inspection, many tube walls in the evaporators were found to experience severe degradations at both sides with distinct corroded defects and general cracking of corrosion layers. Thus, comprehensive investigations including external appearance, microscopic morphology and chemical composition were carried out by using a series of characterization methods. The analysis results demonstrated that the unexpected wall thinning of tubes was primarily ascribed to multiple corrosion factors including uniform corrosion, pitting and interaction behavior between them. Relative failure mechanisms were discussed in detail and prevention measures were also proposed for ammonia evaporators under similar operating condition. [ABSTRACT FROM AUTHOR]

This work presents a sensor fault compensation system, applied to a heat pump's helical evaporator. The mathematical model of the evaporator is given by algebraic and differential equations. These equations were selected according to the phases and regime of the fluid to be evaporated. The sensor fault compensation is based on fault detection and isolation system and a MPC (model predictive control) strategy. The fault detection isolation system is based on a bank of two high-gain observers which have two main purposes. The first one is to generate adequate residuals when a sensor fault occurs. The second purpose is to act as a software sensor, meaning, the measure estimated by the observer replaces the measure given by physical sensor when a sensor fault occurs. The high-gain observers were selected because they are easy to implement and tune. Furthermore, they provide an adequate estimation of the process outputs (when a sensor fault occurs) to a model predictive control (MPC) strategy. The MPC has been implemented to regulate the steam outlet temperature. Several experiments were carried out to show that the MPC regulates the process output even if a fault occurs. The experiments in the evaporator of the absorption heat pump have shown reliability of the method presented in this work to detect a sensor failure, isolate the sensor failure and regulate the steam temperature when a total or partial failure occurs. [ABSTRACT FROM AUTHOR]

The separate type heat pipe heat exchanger is considered to be a potential selection for developing passive cooling spent fuel pool – for the passive pressurized water reactor. This paper simulates the boiling flow behavior in the evaporator of separate type heat pipe, consisting of a bundle of tubes of inner diameter 65 mm. It displays two-phase characteristic in the evaporation section of the heat pipe working in low heat flux. In this study, the two-phase flow model in the evaporation section of the separate type heat pipe is presented. The volume of fluid (VOF) model is used to consider the interaction between the ammonia gas and liquid. The flow patterns and flow behaviors are studied and the agitated bubbly flow, churn bubbly flow are obtained, the slug bubble is likely to break into churn slug or churn froth flow. In addition, study on the heat transfer coefficients indicates that the nucleate boiling is the dominant mechanism in large pipes at low mass and heat flux, with the heat transfer coefficient being less sensitive to the total mass flux. [ABSTRACT FROM AUTHOR]

To investigate the impact of defrosting-water retention on the surface of heat exchangers of the air source heat pump (ASHP) performance during frosting–defrosting cycles, an experimental study has been conducted. Testing was carried out in a controlled-environment chamber with ASHP operational data collected over 16 periodic frosting–defrosting cycles spanning a 360-min period. It was found that the defrosting-water retention undergoes three stages: single-phase water, a mixture of water and ice, followed by a predominance of ice. In this study, the third stage appears after the unit has been in operation for 256 min. A “permafrost area” forms on the lower quadrant of the heat exchanger, which occupies over 20% of the total heat exchanger surface. This area remained continuously occupied regardless of whether the unit was in a frosting or defrosting cycle. The appearance of this “permafrost area” not only impacts the performance of the ASHP, it also indicates the beginning of repetitive “shutdown accidents” for the ASHP units. It is the first time that these phenomena, originally caused by the defrosting-water retention have been revealed. And a quantitative analysis is presented for the resulting performance degradation of the ASHP related to the “permafrost area” conditions and the related “repetitive shutdown accidents”. [ABSTRACT FROM AUTHOR]

• Research on falling-film heat exchangers used in desalination systems, especially multi-effect distillation systems, is critically reviewed to assess the state of the technology. • The presence of non-condensable gasses in desalination systems leads to reduced performance for both condensation and evaporation. • Research on scaling in horizontal-tube, falling-film evaporators in multi-effect distillation systems is insufficient in the open literature. With growing demand for fresh water, there has been an increasing interest in energy-efficient desalination technologies. Falling-film-based, multi-effect distillation (MED) systems have been extensively deployed world-wide. The performance of such systems heavily depends on the efficiency of the evaporation process. In the current paper, research on falling-film heat exchangers used in desalination systems, especially MED systems, is critically reviewed to assess the state of the technology. The review includes falling-film evaporation mechanisms, system operation, and scaling issues. Finally, opportunities for further application and areas where more research is needed are identified. [ABSTRACT FROM AUTHOR]

Abstract: This work presents an experimental and numerical analysis of the flow distribution in the cooling duct of a commercial refrigerated display cabinet. The analysis is carried out on a channel mock-up, in which the air is forced through the evaporator by two fans. A steady-state isothermal numerical analysis of the flow distribution is performed with the commercial code ANSYS-CFX and the k-epsilon method is used to model the turbulence. The numerical results are compared with the 3D air velocity fields, taken on three planes placed at different heights inside the duct, by the stereoscopic Particle Image Velocimetry (PIV) technique. Once validated, the model allows a better comprehension of the flow mal-distribution source and its effect on the velocity field at evaporator exit. [Copyright &y& Elsevier]

A new type of oil-free moving magnet linear compressor with clearance seals and flexure springs has been designed for incorporation into a vapour compression refrigeration system with compact heat exchangers for applications such as electronics cooling. A linear compressor prototype was built with a maximum stroke of 14 mm and a piston diameter of 19 mm. An experimental apparatus was built to measure the compressor efficiencies and coefficient of performance (COP) of a refrigeration system with the linear compressor, using R134a. The resonant frequency for each operating condition was predicted using the discharge pressure, suction pressure and stroke. Refrigeration measurements were conducted for different strokes under each pressure ratio with a fixed condenser outlet temperature of 50 °C and evaporator temperature ranging from 6 °C to 27 °C. The results show that the COPs are around 3.0 for tests with a pressure ratio of 2.5 (evaporator temperature of 20 °C). [ABSTRACT FROM AUTHOR]

Abstract: An experimental investigation of the performance of a wraparound loop heat pipe (WLHP) charged with R134a is reported in this paper. The copper WLHP has a two-pass evaporator section and a two-pass condenser section, each pass is 330 mm in length with an inner tube diameter of 11.2 mm. The considered configuration is similar to that used to build industrial heat exchangers, which are used for heat recovery in energy efficient air handling units. R134a is chosen as the WLHP working fluid as it is widely used. A variable output electrical heater was used to heat the evaporator section with the condenser section being cooled using a 2-pass shell and tube heat exchanger, equipped with 7 baffles to enforce coolant (water) cross flow around the condenser tubes. An average heat pipe thermal resistance expression, based on the experimental data, is introduced and used to characterise the thermal performance for the WLHP under the considered experimental conditions. The nature of the heat transfer processes and the working fluid path within the heat pipe are analysed under various operational conditions and areas of enhancement for this type of heat pipes are identified. An average value for the WLHP overall effective thermal resistances as low as 0.048 °C/W is reported. [Copyright &y& Elsevier]

Abstract: This study experimentally investigated the frost growth on louvered folded fins in microchannel heat exchangers when used in outdoor air-source heat pump systems. The effects of surface temperature, fin geometries, and air environmental conditions were studied. The overall aim was to isolate and quantify the effect of geometry from surface temperature effects. Experimental data of frost weight, local frost thickness, air pressure drop across the coils, time of frost–defrost cycles and heat transfer rates were recorded. Data showed that the frosting time and the frost growth rates depended mainly on the local fin surface temperature. Lower fin density was beneficial because it delayed the blockage of the air flow. The fin length and fin depth had minor effects on frosting performance. The air humidity had a fairly significant effect on rate of frost formation while air velocity seemed to have a small effect on the frost growth rate. [Copyright &y& Elsevier]

Abstract: Laboratory studies of the reaction of steam reforming of light hydrocarbons into methane–hydrogen mixture were performed. Ni- and Ru-containing systems were studied as the catalysts. The design, scale-out, and operation of an APG catalytic reformer integrated with evaporator-heat-exchanger, water condenser and flame burner were reported. Conversion of heavier hydrocarbon components into methane–hydrogen gas mixture exceeded 95–99% during testing the catalytic reformer at 270–360°C. Increasing reaction temperature led to increasing H2 and CO contents in the reaction mixture and complete conversion of LPG and ethane fractions. Both initial and reformed APG were used for fueling a power generation unit on the base of gas internal combustion engine MTES-30. In case of initial APG fueling, the power derating was 22%, exhaust gas contained black smoke. When the power generation unit was fueled by methane–hydrogen mixture produced by APG catalytic reforming, the engine power attained the nominal value; the engine showed excellent dynamic and temperature characteristics, stably supported crank rotation frequency. According to economic analysis concerning different types of power plants with electric power of ∼1000kW, the plants equipped with a catalytic reformer of APG into methane–hydrogen mixture show faster payback of capital investments, as compared to the plants fed by APG directly, due to longer service life, longer overhaul intervals, and low rated power losses. Сatalytic reforming of APG into methane–hydrogen mixture is a promising approach for solving APG utilization problem. [Copyright &y& Elsevier]